The present invention relates generally to radio receivers, and, more particularly, to a radio receiver using baseband filtering in place of conventional, bandpass intermediate frequency selectivity.
The design of conventional superheterodyne receivers is a known art; such receivers typically employ frequency-conversion and filtering circuitry to convert a desired radio frequency (RF) signal to one or more intermediate frequencies (IF) prior to demodulation of a signal received by such receivers.
To illustrate this technique, consider the following example of a receiver designed for the 843 to 870 megahertz (MHz) radio frequency band. This receiver may use a first intermediate frequency ("1st IF") of 55 MHz, and a second intermediate frequency ("2nd IF") of 460 kilohertz (kHz). Such a receiver would contain a bandpass filter to select signals in the 843-870 MHz range, and to reject signals outside such range; a mixer circuit (first mixer) to convert the desired radio frequency signal to a first intermediate frequency of fifty-five MHz; a bandpass filter centered at fifty-five MHz (the "1st IF filter") to select a relatively narrow band of frequencies about fifty-five MHz; and to reject signals outside such range; another mixer circuit (a "2nd mixer") to convert the filtered first intermediate frequency signal to the second intermediate frequency of 460 kHz; and bandpass filtering centered at 460 kHz to select a relatively narrow band of frequencies about 460 kHz, and to reject signals outside that range. The receiver would also contain radio frequency and intermediate frequency amplifiers, where needed, to assure adequate gain and noise figure. The first and second intermediate frequency filter bandwidths would be chosen such that the bandwidth would be appropriate for the type of modulation present on a signal received by the receiver. If the receiver were designed to receive a frequency-modulated signal, the filtered, second intermediate frequency signal would be applied to a limiter, and then to a frequency demodulator. The demodulated signal generated thereby would then be applied to audio processing circuitry (which performs functions such as deemphasis), amplified, and applied to a speaker. The receiver additionally contains local oscillators needed for the frequency conversion process. In this example, the first local oscillator would operate in the frequency range of 788 to 815 MHz, and would be applied to the first mixer; and the second local oscillator would operate at 54.540 MHz, and would be applied to the second mixer. In order to receive a signal at a particular radio frequency, the frequency of the first local oscillator must be set to the appropriate frequency. In this example, the first local oscillator is set to a frequency that is fifty-five MHz below the frequency of the desired radio frequency. Therefore, to receive 843.000 MHz, the first local oscillator is set to 788.000 MHz; to receive 843.100 MHz, the first local oscillator is set to 788.100 MHz; and so forth. The first local oscillator is typically a frequency synthesizer, in order to permit reception on frequency increments of 12.5 kHz in the 843 to 870 MHz band. The second local oscillator is on a fixed frequency, and may typically be a crystal oscillator or another frequency synthesizer.
The receiver in this example could be built using modern component technology. For example, the radio frequency bandpass filter could be constructed using dielectrically-loaded coaxial resonators; the first intermediate filter could be constructed using one or more monolithic crystal filters; the intermediate frequency amplifiers, mixers, limiter, demodulator, audio circuitry, and local oscillator circuits could be combined in one or more integrated circuits, or implemented with discrete components using surface-mount technology, or some combination of the two; and the second intermediate frequency filter could be implemented using one or more modular filters using ceramic resonators. Miniaturized receivers of this type have been constructed using the above technologies, and are characterized by small physical size, low weight, low cost and low power consumption.
Radio receivers have also been designed that use baseband filtering techniques in place of the filtering at the second intermediate frequency in conventional superheterodyne receivers. Such receivers eliminate the need for modular filters using ceramic resonators at the second intermediate frequency, and so make possible further reductions in receiver size and weight.
In a receiver that uses baseband filtering, the signal at the output of the first intermediate filter is down-converted to baseband frequencies by the second frequencies by the second mixer. The output of the second mixer is applied to a low pass filter; and the output of the low pass filter is then applied to a third mixer. The third mixer up-converts the filtered baseband signal to a third intermediate frequency, and this signal is, in turn, filtered, and applied to the appropriate demodulation and audio circuitry. (Further details of the baseband filtering process will be explained in connection with the description of the preferred embodiments of the present invention to be described hereinbelow.) The remainder of the receiver circuitry in a receiver using baseband filtering is much the same as it is for a conventional superheterodyne receiver.
The process of down conversion to baseband, low pass filtering, and up conversion ideally results in a circuit having a bandpass filter characteristic. The input frequency is centered at the first intermediate frequency, and the output frequency is centered at the third intermediate frequency.
Problems can arise, however, when the low pass filter is implemented as an active filter, as is the case when the second mixer, low pass filter, and third mixer are combined into an integrated circuit. The active filter used as the low pass filter consists of resistors, capacitors, and operational amplifiers. The operational amplifiers used may contain a DC offset at the output thereof, and this must be eliminated before the filtered baseband signal is applied to the third mixer. To block such DC components, blocking capacitors of appropriate value are used in the low pass filter to create an AC-coupled, low pass filter.
Typically, portions of the low pass filter circuitry also have associated resistive impodances in a shunt connection with respect to the series blocking capacitors. Such a resistive-capacitive combination has the characteristics of a high pass filter, which attenuates very low frequency components in the baseband signal. This is an undesired effect resulting from the removal of the DC component. Portions of the baseband signal can be attenuated by the high pass effect if sufficiently low in frequency. Furthermore, frequency components which are low in frequency in the baseband signal appear as frequency components near the center of the third intermediate frequency after up conversion. This combination down conversion, low pass filtering with an AC-coupled, low pass filter, and up conversion results in a circuit with a bandpass frequency response having a notch in the center, rather than the ideal bandpass characteristic. Significant distortion of the demodulated output signal results if significant baseband signal power is lost in the notch. For example, if an FM signal having a low modulation index passes through the circuit, and the carrier of the signal falls in the passband notch, then the output signal is highly distorted.
What is needed, therefore, is means for preventing significant attenuation of low frequency components in the baseband signal by the AC-coupled, low pass filter, and thereby avoiding significant distortion in the demodulated output signal, in a receiver using baseband filtering to achieve intermediate frequency selectivity.